This invention relates to ring laser gyroscopes in general and more particularly to a method and apparatus for preventing lock-in in a ring laser gyroscope.
A typical ring laser gyroscope is disclosed in U.S. Pat. No. 3,373,650, to J. E. Kilpatrick. Such a ring laser gyroscope consists of a triangular optical resonator capable of supporting a clockwise and a counterclockwise beam nominally of the same frequency. When such a device is rotated, the frequency in one beam increases while the other decreases. When the beams are combined the frequency shifts to produce a beat note frequency proportional to the input angular velocity which can then be detected, and likewise is a measure of angular rate.
However, at low input rates, light scattered from the mirrors in the triangular optical resonator tends to couple the two beams together so that the beat note, and hence the output is suppressed. This effect is known as lock-in. To overcome this problem various methods of biasing the ring laser gyroscope to avoid lock-in have been proposed A number of these methods include a mechanical biasing in which the ring laser gyroscope is physically rotated at a controlled rate so as to prevent lock-in. Other methods of biasing utilize phase shifting of the beams. For example, such phase shifting can be done with different types of nonreciprocal phase shifters such as a Faraday cell or a Pockels cell. The aforementioned U.S. patent to Kilpatrick creates a bias by the means of a Faraday cell.
Another solution to this problem utilizing two separate cavity resonances is described in U.S. Pat. No. 3,879,130. This patent, in its background portion includes a detailed description of the various prior art methods used in overcoming a lock-in problem. In U.S. Pat. No. 3,627,422, biasing is taught using an acoustic generator, light shutter or electro-optical crystal. The aforementioned Pockels cell is a device of this type. Another device utilizing the Faraday effect is disclosed in U.S. Pat. No. 3,392,622. A device which utilizes the Zeeman frequency offset effect for biasing is disclosed in U.S. Pat. No. 3,382,759.
A common characteristic of these devices using non-reciprocal phase shifters for biasing the laser is that the phase shifting is carried out in the main optical path. Only a very small phase shift is required. For example, a phase shift of 5 arc seconds between the two beams will cause a 10,000.degree./ hour bias using typical ring laser gyro parameters. The problem with the conventional phase shifters such as the aforementioned Faraday cells is their extreme sensitivity to environmental effect such as temperature gradients and stray magnetic fields. These effects can readily cause unpredictable phase shifts of a fraction of a second of arc which results in an unacceptably large bias variation. Thus, there is a need for an improved manner of carrying out a non-reciprocal phase shift to bias a ring a laser gyroscope to avoid lock-in which does not suffer from these disadvantages, i.e., which is not particularly sensitive to environmental effects.